[J0sh...]

PHP код:

public OnPlayerUpdate(playerid) {
    new targetplayer = GetPlayerTargetPlayer(playerid);
    if(GetPlayerTeam(targetplayer) == GetPlayerTeam(playerid) && targetplayer != INVALID_PLAYER_ID)
    {
        GameTextForPlayer(playerid, "~R~do not shoot at team-mates!", 5000, 3);
    }
} 


Source: https://sampwiki.blast.hk/wiki/GetPlayerTargetPlayer

Second Source for actors: https://sampwiki.blast.hk/wiki/GetPlayerTargetActor